Several resistant microorganisms forming protective cysts, such as Giardia and Cryptosporidium, have been found in drinking water systems in recent years and have demonstrated resistance to conventional chlorine disinfection. The conventional disinfection process involves free chlorine (HOCl/OCl−) as a single disinfectant or monochloramine (NH2Cl) as a single disinfectant to inactivate microorganisms during drinking water and wastewater treatment.
To solve the problem of microorganisms resistant to conventional chlorine disinfection, the drinking water industry has looked for alternative disinfectants, such as ozone (O3) and chlorine dioxide (ClO2). These alternative disinfectants are believed to be more effective in the process of inactivating microorganisms that are resistant to free chlorine (HOCl/OCl−) or monochloramine (NH2Cl) disinfection. However, ozone and chlorine dioxide produce disinfection byproducts that may be considered detrimental to human health.
Ozone produces brominated organic disinfection byproducts and bromate (BrO3−) when the source water contains bromide (Br−). Ozone also oxidizes chloride (Cl−) present in the raw water to produce chlorinated organic byproducts. In addition, ozone disinfection is an energy intensive process and requires sophisticated operational skills. Ozone is not suitable as an alternative disinfectant for small water treatment systems, which represent about 87% of community water systems in the United States.
Chlorine dioxide (ClO2) disinfection produces byproducts such as chlorite (ClO2−), chlorate (ClO3−), and organic byproducts. If the water disinfected by chlorine dioxide is stored in uncovered storage basins, sunlight increases the chlorate concentration.
The present dilemma facing the drinking water industry is to find a disinfection strategy that will strike the balance between the effectiveness of inactivating pathogen organisms and the minimization of byproducts formed during water disinfection. In this regard, neither of the alternative chemicals to chlorine disinfection nor free chlorine and monochloramine as presently used in the drinking water industry can deliver on the above dilemma.
New regulations require finished water to be free from pathogens and contain the minimum possible of disinfection byproducts, so the water industry is turning to the use of multiple disinfectants. However, the trend in the practice of multiple disinfectants presently established is the sequential use of a primary disinfectant that essentially does the actual inactivation work, followed by a secondary disinfectant that provides a residual disinfectant in the distribution system and may minimize the byproduct formation. Even with the sequential use of disinfectants involving ozone, free chlorine, monochlramine, and chlorine dioxide, the problem of byproduct formation is not solved.
Traditionally, monochloramine (NH2Cl) and dichloramine (NHCl2), which are commonly called chloramines or combined chlorine, are considered weak disinfectants and weak oxidants compared to free chlorine. This makes free chlorine the only effective disinfectant. Accordingly, chloramines are considered byproducts that need to be destroyed to guaranty an effective disinfection. This consideration is embedded in a well-known disinfection practice called breakpoint chlorination that aims at complete oxidation of ammonia to prevent the interference of chloramines in the disinfection process. The problem with breakpoint chlorination is the quantity of chlorine used to achieve either partial or total destruction of ammonia: it takes a minimum of 7.6 g (grams) of chlorine to destroy 1 g of ammonia dissolved in the raw water under treatment. The chlorine reaction is not restricted to ammonia only during the process, so there is enough chlorine to react at the same time with dissolved organic matter to form undesirable byproducts.
Disinfection systems typically use a plug flow type of reactor configuration alone. Chlorine is bled into the system and allowed to drift downstream. During this process, chlorine is mixed slowly with the total mass of water to be disinfected. This favors byproduct formation because there is enough time for extensive chlorine reactions to occur with dissolved organic substances.
Conventional disinfection practices also apply to wastewater effluent disinfection. This requires a high dose of chlorine to reach the effluent discharge standard for E. coli, so an excessive amount of chlorinated organic byproducts are formed during the process. Given the high density of suspended solids and the potential of high chlorine demand in the wastewater effluent, chlorine is ineffective in inactivating microorganisms forming protective cysts that may be found in wastewater.
To solve the problem of chlorinated organic compound formation, the wastewater industry has turned to ultra violet (UV) light disinfection as an alternative to chlorine. However, the inactivation capability of UV disinfection against microorganisms forming protective cysts, such as Giardia and Cryptosporidium, is very limited. Under some circumstances, the photobiochemical damage to organisms caused by UV irradiation can be repaired through photoreactivation or dark repair. This allows microorganisms inactivated by UV to regain their viability following the disinfection process. Additional problems that may impair the effectiveness of UV disinfection include limits on effective lamp output and the presence of suspended solids in wastewater. Therefore, conventional chlorine and UV disinfection are inadequate to effectively inactivate microorganisms including protozoan cysts in wastewater effluent.
Accordingly, it would be desirable to have a water disinfection system using simultaneous multiple disinfectants that overcomes the disadvantages above described.